A new hypothesis is advanced to explain the unusual phenomenon of ball lightning. The properties of ball lightning are described and the previous hypotheses are discussed. The hypothesis presented here assumes the existence of an unusual state of matter that is still under study. The principal body of ball lightning is a luminous hot plasma. It is assumed that a nonplasma core is a metallic condensate of nitrogen and oxygen atoms with the properties of a hard body and that the high specific energy of ball lightning is related to the internal energy of the phase state of this condensate. The doughnut-shaped core will be held in the plasma by its own magnetic field if a strong current circulates in it. The long duration of ball lightning indicates that the substance of the ring is probably in a metastable and superconductive state.